How Do I Control Humidity in My Greenhouse?

evaporative cooling system

Managing and balancing temperature and humidity in the greenhouse can be a source of frustration, further compounded by location, climate, greenhouse environmental control systems, and crop choice. Before considering options like venting, humidifiers, and evaporative cooling walls, it is important to understand relative humidity and its relation to temperature. Once this is understood we can take a look at solutions for managing both excess and inadequate humidity. 

What Is Relative Humidity?

Relative humidity is the amount of water vapor in the air, compared to how much the air can hold at the same temperature. It is expressed as a percentage. So a relative humidity of 25% means the air is holding a quarter of the water vapor it is capable of holding. At 100% relative humidity, the air can’t hold any more moisture and water will condense on different surfaces (leaves, walls, etc.).

What Is The Relationship Between Relative Humidity And Temperature?

As temperature increases, the amount of water vapor that the air can hold increases (as shown below). As seen in the graph below, if the temperature is 80F degrees and the relative humidity is 42%; then for the same absolute humidity, if the temperature is 55F degrees the relative humidity is 100%. While the absolute humidity stays constant, the colder air can not hold the same amount of water vapor, therefore the water condensates.

Relative versus absolute humidity

Relative humidity is related to temperature, and vice-versa, combined it’s what we call Vapor Pressure Deficit (VPD). Plants know this and that’s what drives their transpiration and health. We sometimes measure Vapor Pressure Deficit (VPD) to know if the combination of a given temperature and relative humidity is healthy for the plants. 

Read more about VPD in our blog “An Introduction to VPD” here.

As temperatures increase, higher levels of humidity becomes more tolerable for plants. Conversely, as it gets cold, the amount of moisture becomes a problem more quickly. Therefore, it is important to make sure your greenhouse is not too cold.

Now that we have a better understanding of how humidity is related to temperature, we can better assess how to manage it in the greenhouse environment.

How Do I Deal With Excess Humidity In The Greenhouse?  

One option is to exhaust the moisture out by venting. The limitation with this method is that it requires bringing in outside air. In freezing or very cold conditions, the outside air can stress the plants if they are directly hit by this cold, dry air flow. Having a relatively small amount of air exchange will allow for mixing both warm and cold air, reducing the risk of stress. 

Another option is to add a dehumidifier to your climate control. The dehumidifier is designed to condensate the water vapor in the air. A condensate line drains out the water and reduces the relative humidity levels in the greenhouse. It allows for keeping the greenhouse entirely sealed: no waste of energy, and no waste of CO2 (if you are supplementing with CO2).

How Do I Deal With Insufficient Humidity In The Greenhouse?

As mentioned before, relative humidity is related to temperature and how much moisture the air can contain. In a given volume of air, the warmer, the dryer, the cooler, the more moist. If the relative humidity is too low, it might come from an excessive temperature. A first step might be to try and cool the greenhouse space. 

The only other way of increasing the relative humidity is by physically adding water. It can be done through a misting or fogging system, standalone humidifiers, or using an existing evaporative cooler (wet wall) without venting. Try to avoid spraying water on corners or “dirty” surfaces as it will increase the risk of mold and pest inside your space. 

outside wet wall

Still Have Questions About Dealing With Excess/Insufficient Humidity In Your Greenhouse?

Check out The Year Round Solar Greenhouse Book

Or contact us here

lettuce in the greenhouse

Graph Source:

Eclimate Notebook:

Recent Posts


Like This Story?

Get more articles and incentives through our newsletter!

By clicking the button above, you agree to receive emails from

Unsubscribe at anytime.

Related Posts

Tell Us About Your Project